Gas fired hot air furnaces have long been used to heat spaces in both residential and commercial setting. Most conventional gas fired furnaces include a plurality of heat exchangers spaced apart to allow air flow therebetween. The heat exchangers define an internal flow path for hot combustion gases supplied by burners. Heat transferred through the heat exchangers may be used to effect heating of a particular area. The furnace works by sending hot combustion gases through the heat exchangers and blowing room air over the heat exchangers so as to heat the air from the furnace into the area to be heated.
In order to control the air temperature of the hot air exiting the furnace and into the room, you control the temperature of the heat exchangers. This is typically done by controlling the hot combustion gases flowing through the heat exchanger. An increase or decrease in the combustion gases can be affected by controlling the combustion flame exiting the burner. A known burner arrangement is shown and described in U.S. patent application Ser. No. 10/299,479, filed Nov. 19, 2002, entitled “One Shot Heat Exchanger Burner”, status of which is allowed. This application is published as US2003/0101983 A1 on Jun. 5, 2003, and incorporated by reference herein for all purposes.
As schematically shown in FIG. 1, this burner assembly includes a burner 10 defining a burner face 12. The burner face is spaced in close proximity to a plurality of heat exchangers 14. A gas air mixture is fed through a conduit 16 into the burner 10 where it is ignited at the front face 12 thereof. The flame 16 produces combustion gases which enter the heat exchanger as shown by arrows A. Room air may be blown across the heated heat exchangers as indicated by arrow B to heat the air exiting the furnace.
It may be appreciated that regulation or modulation of the fuel air mixture entering the burner can control the flame and thereby the temperature of the heat exchangers. It has been found that using burners of the type shown in FIG. 1 you can modulate a fuel air mixture at a 2:1 ratio, i.e., you can increase or decrease the fuel flow between 100% and 50% of capacity. Any attempt to regulate the fuel flow to less than 50% of capacity could result in combustion problems such as a generation of high CO levels. Thus, in conventional burners, an attempt to regulate the temperature of the heat exchangers so as to maintain exiting air temperature at a controlled set point temperature results in the need to frequently cycle the burner between an off and on position. Such frequent cycling results in a range or band width of the set point temperature being within an undesirable range of 10°.
To reduce such frequent cycling, the prior art has also seen the use of multiple burners in a single furnace. Multiple burners allow cycling among one or more burners so as to increase the modulation. However, the use of multiple burners in a single furnace is not a cost effective solution. Also, even in multiple burner situations, frequent on/off cycling results in heat exchangers seeing both hot and cold temperatures. When a heated heat exchanger cools, it forms undesirable condensation within the internal cavity of the heat exchanger. Any contaminants in the air, when condensed, can form acids which reduce the life of the heat exchanger.
It is, therefore, desirable to provide a fuel fired furnace which allows increased modulation without known undesirable effects and without the need to employ multiple burners in a single furnace.